Copy the URL of the YouTube video thumbnails which would you like to download
put the URL of the YouTube video into the input and then press the "Show the thumbnail" button.

Copy the ID of the video thumbnails which would you like to download and
put the ID of the YouTube video into the input and then press the "Show the thumbnail" button.
Example: the URL of the video is http://www.youtube.com/watch?v=7mEeqs_zXwo
the video ID is 7mEeqs_zXwo;

When the page is loaded make a right click on the one of the nine different picture and choose Save image as... it's so easy :)